Known embodiments of exhaust-gas recirculation (EGR) conduct an exhaust-gas train branched off from the engine, on the exhaust-gas side, for recirculation into the fresh-air path of the engine, within an EGR section. They comprise a cooling device connected with the cooling system of the engine as an exhaust-gas/coolant heat exchanger, as well as an EGR valve, the task of which consists in regulating the recirculated amount of exhaust gas with adaptation to the characteristic field data of the engine.
In this regard, reference is made to DE 10 2010 014 845 A1 as an example.
On the one hand, an important concern in connection with diesel engines is to utilize exhaust-gas recirculation (EGR) for NOx reduction. In this regard, exhaust gas that is as cold as possible is to be supplied to the engine on its fresh-air side, in order to keep the process temperature as low as possible for the stated purpose. For this reason, it is provided, according to a known embodiment, to dispose the EGR valve on the cold side of the heat exchanger, but this brings with it the disadvantage that the EGR valve tends to accumulate soot at operating points below the condensation point of the exhaust gas.
In the case of another known embodiment, the EGR valve is therefore disposed on the hot side of the heat exchanger, but this causes it to be subject to great thermal stress, and therefore it only achieves the limited useful lifetime that is similar to a component subject to wear.
In addition, cooler aging (fouling) of the heat exchanger caused by surface deposits is an additional problem, leading to deterioration of the heat transfer in the cooling section, and this has a disadvantageous effect on the method of action of the EGR section.